At the recent DEFCON hacking conference, security researchers demonstrated a method to crack the MS-CHAPv2 authentication protocol with a 100% success rate. MS-CHAPv2 is used as the default authentication method for remote access VPN in Forefront TMG 2010.
With the public availability of tools to automate the cracking process, PPTP communication using MS-CHAPv2 should be considered unencrypted. There are two options available to mitigate this concern: disable MS-CHAPv2 and enable EAP with PPTP, or disable PPTP and switch to a more secure remote access VPN protocol such as L2TP/IPsec or SSTP. Enabling EAP requires the use of smart cards or certificates for authentication which makes implementation more challenging. SSTP is an excellent option as it leverages SSL/TLS to protect the MS-CHAPv2 authentication process. However, SSTP is only supported on Windows Vista SP1 and later clients. L2TP/IPsec is another good choice, and although it does support certificates it can also be configured using a pre-shared key. If long, complex passwords are used and care is taken to ensure that the password is well protected, it can provide a secure remote access solution.